In most X-ray installations, the X-ray detector is coupled to other components of the X-ray installation. Thus, in the case of overtable systems, for example, the X-ray detector is located inside a patient positioning table and can be moved there in various directions below the table surface.
In facilities where the patient is X-rayed while standing, the X-ray detector is generally located in or on an appropriate wall stand. In undertable systems, the X-ray detector is suspended from a ceiling stand fastened above the patient positioning table such that it can be positioned at a suitable point above the patient positioning table. In the case of these systems, the X-ray source is likewise arranged on a suitable stand or is located in the case of an undertable system below the patient positioning table so that the X-ray source can always be positioned in an appropriate fashion relative to the X-ray detector and to the examination object so that the region of the examination object to be recorded is located in the beam path between the X-ray source and X-ray detector.
Moreover, there are, for example, so-called “C arm units” where an X-ray source is arranged on one end of a C-shaped support that can pivot about the patient. Here, the X-ray detector is arranged in opposite fashion on the other end of the C-shaped support.
However, there are also mobile X-ray detectors in addition to these stationary systems. A classic example of this is a film/foil detector or a storage foil detector. Such a mobile X-ray detector is not connected to other components of the X-ray system in any way and can be positioned at will by the user.
Such detectors are used, for example, whenever it is impossible, or possible only with difficulty, to transport the patient to the X-ray unit or to reposition him/her on a patient positioning table for example when he/she is under intensive medical care. In this case, the mobile X-ray detector is, if appropriate, positioned in the patient's bed at the appropriate point, and an X-ray source, fastened on a moveable stand, with an x-ray generator is arranged in an appropriate fashion relative to the region to be examined and relative to the X-ray detector. Then an X-ray picture is taken in situ.
It must be ensured with all X-ray detectors that the desired pictures are respectively taken with a specific dose, in order on the one hand to keep the radiation burden on the patient minimal as far as possible, and on the other hand to obtain an X-ray image of optimum mean optical density. As a rule, use is made for this purpose of a dose regulating device or a so-called automatic exposure control (AEC) that determines the incoming dose and switches off the X-radiation once a specific dose has been reached.
In this case, dose measuring elements are used that measure the X-ray dose rate incident at the X-ray detector. A signal that is proportional to the dose respectively impinging is obtained by integrating this X-ray dose rate over the recording time.
Various types of sensor can be used as dosimeter. It is, for example, customary to use ionization chambers. A current proportional to the dose rate is generated in an ionization chamber by the X-radiation in an air capacitor charged with 300 000 V, for example. Such ionization chambers are classically arranged, inter alia, upstream of a film/foil detector.
Further sensors suitable as dose measuring element are so-called semiconductor radiation receivers. Such a semiconductor radiation receiver is not transparent to radiation, and is therefore used downstream of the detector. Alternatively, it is also possible to use photomultipliers, electron multiplier elements or photodiodes that indirectly measure the dose rate via the optical brightness of an image intensifying output screen.
The dosimeters are permanently installed in the X-ray facilities, specifically in such a way that the X-ray detector is arranged downstream or upstream of the dose measuring element in accordance with the type of X-ray detector and the type of dose measuring element. Thus, for example, the dosimeter is integrated on a C arm unit at the appropriate end of the C support, directly in the housing of the X-ray detector.
The same holds, for example, for X-ray detectors suspended from ceiling stands, or X-ray undertable systems. In the case of overtable systems or systems with a wall stand, an appropriate automatic exposure control in the form, for example, of a so-called “catapult Bucky table”, is permanently installed in the patient positioning table or in the wall stand.
When use is made of film/foil cassettes, the latter can be pushed into the catapult Bucky table such that they are arranged in an appropriate fashion relative to the dosimeter, that is to say directly upstream or downstream of the dosimeter depending on the type of the dosimeter in the beam path. The dosimeter is connected via control and data cables inside the table or stand to a control unit in which the signal measured by the dosimeter is then appropriately integrated in order to detect the dose respectively currently impinging, and to switch off the X-ray machine at the correct point in time.
These automatic exposure controls cannot be used when mobile detectors are used. Consequently, such recordings must be carried out in the form of a so-called “free exposure”. Here, the user must work out in advance the exposure time after which the correct dose has approximately been reached, and must then ensure that the X-ray machine is switched off appropriately under time control. The operator requires considerable experience to calculate or estimate the correct exposure time. Faulty exposures therefore frequently arise with such free exposures, and so recordings have to be repeated. This leads to a higher radiation burden on the patients, and to unnecessary consumption of material.